Indeed! I think the main issue is not speed, but that Nvidia GPU were not working on some MB because oft he 3.0 Riser. So you have to plug it directly, set the PCIe speed in BIOS to 3.0, and THEN use the riser.I would be very interested to see some (real life / gaming) performance tests using the same hardware running on PCIe Gen 3 vs. Gen 4. If it's just a few FPS difference it's not (yet) worth to upgrade the riser IMHO.
Edit: just found some comparisons. Only 0 to 3% difference in FPS depending on the test. Guess I'll wait for RTX4XXX until I upgrade to PCIe Gen 4.
Is more compatibility than speed (I never seen ANY difference even between PCIe 3.0 x8 and x16 with my Titan V while rendering...)